1. Field of the Invention
The present invention relates to a light-emitting device which includes an insulating member, a pixel portion and driver circuits for supplying signals to the pixel portion, and in which the pixel portion and the driver circuits are formed on the same insulating member. Specifically, the present invention includes techniques effective in improving a device having an element constituted of a pair of electrodes and a thin film of a light-emitting material interposed between the pair of electrodes (which element will be hereinafter referred to as “light-emitting element”) (which device will be hereinafter referred to as “light-emitting device”). The light-emitting device of the present invention covers an organic electro-luminescent (EL) display and an organic light-emitting diode (OLED).
In particular, the present invention includes techniques effective in improving a device having an element constituted of an anode, a cathode, and a thin film of a light-emitting material capable of electroluminescence and interposed between the pair of electrodes (which thin film will be hereinafter referred to as “EL film”) (which element will be hereinafter referred to as “EL element”) (which device will be hereinafter referred to as “EL light-emitting device”)
Light-emitting materials usable in the present invention comprise all of light-emitting materials capable of emitting light (phosphorescence and/or fluorescence) by singlet excitation, triplet excitation, or both of singlet and triplet excitation.
The present invention can also be applied to a device having an element in which a liquid crystal material is interposed between electrodes (which element will be hereinafter referred to as “liquid crystal element”) (which device will be hereinafter referred to as “liquid crystal display device”).
2. Description of the Related Art
In recent years, the development of active-matrix EL light-emitting devices have been promoted. In active-matrix EL light-emitting devices, thin-film transistors (hereinafter referred to as “TFTs”) are provided in each of pixels (EL elements) of the pixel portion, and the current caused to flow through each EL element is controlled through the TFTs to control the luminance of the pixel. Therefore, voltages can be uniformly supplied to the pixels even if the number of pixels to be formed by the pixel portion is increased. For this reason, active-matrix EL light-emitting devices are suitable for forming a high-definition image.
Active-matrix EL light-emitting devices also have the advantage that circuits including a shift register and a latch or a buffer constituting driver circuits for transmitting signals to the pixel portion can be formed by TFTs on one insulating member on which the pixel portion is also formed. Therefore, when an EL light-emitting device of this structure is manufactured, it can be designed so as to be remarkably small in size and weight.
Active-matrix EL light-emitting devices, however, have a drawback in that a complicated TFT fabrication process increases the manufacturing cost of the device. Moreover, since a plurality of TFTs are formed simultaneously, the manufacturing process may be so complicated that it is difficult to ensure a satisfactory yield. In particular, an operation failure in the driver circuits may result in a line defect such that one row of pixels do not operate.
FIGS. 18A and 18B show the basic structure of an active-matrix EL light-emitting device. Referring to FIG. 18A, a TFT 1802 for controlling a current flowing through an EL element (hereinafter referred to as “current control TFT”) is formed on a substrate 1801, and an anode 1803 is connected to the current control TFT 1802. An organic EL film (thin film of a light-emitting organic material capable of producing electroluminescence) 1804 and a cathode 1805 are formed on the anode 1803. Thus, an EL element 1806 constituted of the anode 1803, the organic EL film 1804 and the cathode 1805 is formed.
In this light-emitting EL device, light produced in the organic EL film 1804 passes through the anode 1803 to travel in the direction of the arrow indicated in the figure. The current control TFT 1802 acts as a shielding such as to block light emitted to travel to an observer and to cause a reduction in the effective emission region (a region through which the observer can observe emission of light). If the effective emission region is reduced, a need arises to increase the intensity of light emitted from the organic EL film in order to obtain a bright image. This can be achieved by increasing the voltage at which the organic EL element film is driven. However, if the drive voltage is increased, there is apprehension that the degradation the organic EL element film is promoted.
An active-matrix EL light-emitting device of a structure such as shown in FIG. 18B, designed to solve this problem, has been proposed. Referring to FIG. 18B, a current control TFT 1807 is formed on a substrate 1801, and a cathode 1808 is connected to the current control TFT 1807. An organic EL film 1809 and an anode 1810 are formed on the cathode 1808. Thus, an EL element 1811 constituted of the cathode 1808, the organic EL film 1809 and the anode 1810 is formed. That is, the structure of the EL element 1811 is in an inverse directional relationship with that of the EL element 1806 shown in FIG. 18A.
In this active-matrix EL light-emitting device, most of light traveling to the cathode 1808 side after being produced by the organic EL film 1809 is reflected by the cathode 1808 to be emitted through the anode 1810 in the direction arrow indicated in the figure. Therefore, the whole of the region where the cathode 1808 is formed can be used as an effective light-emitting region, so that an active-matrix EL light-emitting device having a high light extraction efficiency can be obtained. Further, even if the drive voltage is low, a high intensity of emitted light can be obtained to provide a bright image.